Thursday, 14 February 2019

Project Round Up

Several projects are now either coming to completion or entering a more technical phase. The QRP Labs VFO kit just requires a little reshaping of its case and the addition of a suitable regulator for its 5v supply. I only have a single 7805 device spare, and these waste a heck of a lot of power as heat, so am looking instead to use a Buck converter here. I happen to have the PCB from a broken (physically) USB car charger, which I think will do nicely.

With the VFO complete, I can then start to experiment with the PRC-349s!

The QRP Labs 5W PA kit is now complete save for testing and alignment. I think I can get away with setting up the bias on the bench, but the rest might need to wait for its installation into the U3S case.

5w PA prior to adding the power devices
Modifications to the U3S include the addition of an RF changeover relay and another RF connector for the switched through antenna to feed a receiver, plus perhaps a couple of auxiliary relays, and new firmware. But before that, the QLG1 GPS has to be mounted into its specially 3D printed box and bolted to the top of the U3S case! Many thanks to my colleague Alex for that.

The PRC-344 Scan Control project has completed design phase, and now requires software! This means ive to learn to code in C for a PIC16F887! Im no coder so this is probably going to be the most difficult part of the project! I'll get the basic 'Blind Scan' system working first, before considering the remote control and battery monitor functions - all of which require some very clever features! Ultimately though learning the coding will lead to the remote VFO controller for the PRC-320!

Ive also got to repair my ladders, for which a number of 100mm M5 bolts are on order. But before ANY of this work can be completed, I have to finish replacing the mains distribution in the workshop, oh and put a couple of wines on...

Thursday, 7 February 2019

Louder Whispers

I have to say, again, I really do like the kits Hans Summers designs and produces, and so have now begun the 5W HF PA, to add to my U3S beacon. Despite the very compact PCBs, construction of QRPlabs kits is amazingly straightforward, as Hans really does guide you through step by step with superbly detailed assembly instructions.

Along with the 5W PA kit, I also ordered a new MCU for my U3S with the latest firmware, as this supports the external switching functions I intend to use to add an antenna changeover relay to give an ongoing antenna feed out to a receiver - in my case an FRG-100.

But enough! You dont want to hear me chatting about this, so, here are the pictures of the build so far...

The bagged kit. Good strong bags!

Capacitors fitted. The quality of the PCB can be seen here

Fitting the resistors. This kit is rather resistor heavy!

All the 1/8w resistors fitted except one (a later mod)
 This stage was the only time I had any mishap with tonights part of the build, when I inadvertently let solder flow into an adjacent hole. Rather than go to the trouble of clearing the whole, I simply positioned the leg of the resistor that was to go into it, and melted the solder while gently pushing the resistor in, so it slid cleanly down and through.

The 2W 0.33r resistor and the preset installed
5V regulator and IC socket now fitted
Thats it for tonight! Im going to spread this build out over a few days. Hopefully, by the time I complete this, including installing and connecting it up inside the U3S case, I will also be in a position to make a final good job of the QLG GPS receiver that governs the beacon. A work colleague is into 3D printing, and has offered to print me a case to my specifications for the QLG, which will bolt onto the top of the U3S case, finally making a neat and tidy all in one job of it.

Before I get too far into the semiconductors, I really should finish the rewire of the mains distribution for the bench! The 4-way socket is being replaced with a 6-way. And the earth pins on the ESD plugs, which is what has damaged the safety shutters on the 4-way panel, are going to have a bevel filed into them! At least the workshop lighting has now been replaced with the new LED strips. If only now I could get used to the new location of the light switches!

Monday, 4 February 2019

SMT and PCB design

The projects im currently working on, if im to make them fit into the equipment they are designed for (ie, not just remain 'bench experiments') require converting to compact PCBs using SMT parts.

Designing PCBs properly requires following a few special rules. Sadly, the auto-routers provided with most PCB CAD packages fail to follow these rules properly. One of these rules is that the use of blind vias (that is through board connections without a through hole component legs) should be kept to an absolute minimum.

This leads to a big problem with the sort of ultra compact, digital electronics im working with. Up until now, the top copper of a PCB has, in most of my work, been strictly RF groundplane. Now, I find im having to work with both top and bottom copper as signal lines, as well as power and ground connections. Add into this the additional rule that top copper tracks cannot pass across IC pin pads except where the package is large enough, such as a SOIC, then any auto-router naturally creates multiple blind vias.

So I find myself manually routing PCB layouts, not on the CAD packages, but on paper, with the help of several coloured pens. By playing with the orientation of the IC packages, eventually it is possible to come up with an effective layout that obeys the rules.

Friday, 1 February 2019

PRC344 Scan Project, further design work

Ive spent many hours this last week working on the design details for this project, and each time it has come down to a toss up between physical size and number of IO ports.

While working out the details, the need to draw virtually no current from the radio was paramount. Originally, the plan was for zero current, as all the control electronics would be isolated, but a flaw in the radios original design has led to a change of perspective - the PRC344 has no battery low warning!

Now, PIC chips can be put into sleep modes where they will draw negligible current, in the order of a few micro-amps, yet still react of external or some internal interrupts. So I started thinking of ways to monitor the battery with the MCU, and to set up a low battery warning. But this meant that the MCU would have to be powered all the time, so the search extended to 3v regulators that not only could cope with a 24v input, but also had extremely low quiescent currents. This in turn led to finding serial line drivers that also had ultra low current sleep modes!

But all that was irrelevant compared with another small problem. The MCUs I had chosen did not have enough IO ports to run the system, and so required port expanders. These extra chips need controlling over a serial data link such as SPI or I2C. As im trying to design this to use materials I already have, I found my lack of coding knowledge a major disadvantage, as none of my devices has SPI, only I2C, and none of my stock of preferred MCUs can do I2C natively. This meant I would have to bit-bang the entire instruction set for the port expanders from scratch!

One device I do have though, which can not only do SPI or I2C through native hardware, but also has sufficient IO ports to mean I dont even need the port expanders, is the PIC16F887. But I had originally discounted this device due to its size - a 40pin DIP!

But the realisation came to me that since I was trying to make this a very small unit, and so was looking at SMT parts for almost everything, I should look at the SMT packages for this chip - and the 44pin TQFP package is only 10mm square yet the legs are relatively accessible!

So, a redesign using the 16F887 has resulted in a system that has just 6 ICs (including a sophisticated LDO regulator and RS232 line driver), will fit in the space left by disabling the radios transmitter, and can monitor the battery voltage during sleep mode to bring the entire systems quiescent current draw down to just a few tens of micro-amps!

...oh, and the code now is little more than beginner level port on/off stuff!

I'll start prototyping on the weekend, first testing out basic port driving code and E2PROM stored look-up tables. Sadly I cant prototype with the HV multiplex chips yet as I cant find my SMT break-out PCBs, and have had to order more.

Sunday, 27 January 2019

PRC344 Scan Project

A drunken challenge accepted means that I am in the process of developing a controller that will give the Clansman PRC344 radio a scan capability. This is pretty frivolous to say the least! - the radios 50kHz steps means many active frequencies will never be heard anyway! But, its a fun thing to try and make happen,

The idea is to develop a means of controlling the tuning of the radio in timed steps, with received signals halting the scan. Initially, the system will be 'blind', that is, entirely self contained within the radio with no functions other than to start and stop it, and to override the halt to continue scanning. But later, by sacrificing one of the radios audio connectors, and external controller could be attached for more sophisticated operation.

This is no easy task. For a start, I have to take control of the synthesiser, this means 14 control lines plus a means to break the ground connection to the frequency dials. But, I also need to override the radios RF front end tuning, which means taking control of another 18 lines, and switching over a further three! I also need to accept control inputs from 3 different parts of the radio! Plus, I need an RS232 port. All in all, that means arranging for a minimum of 38 control lines on my controller!

And all this has to not only run on 3V to be compatible with the synthesisers logic, but switch tuning voltages up to 70V! And, when not selected to scan - have zero current drain!
 If all thats not bad enough, the entire thing must fit within the space left by the removal of the Tx control module!

Yet, I think I have the answer! And its all down to modern high performance microcontrollers and sophisticated logic chips. A string of resistors, a couple of 24v DPDT relays, two MOSFETs, a zener, a 3v regulator, three high voltage 8-port multiplexers, two 16-bit I2C port expanders, and a tiny little biddy PIC microcontroller! All built with SMT parts.

Just waiting on the prototyping samples to arrive. In the meantime, I have to do the daunting part that will form the heart of the system - Ive to learn to program in C...

Saturday, 26 January 2019

QRPlabs Si5351A VFO kit

Once again today I have had the pleasure of building one of Hans Summers kits. Hans designs and sells, as QRPlabs, incredibly versatile and high quality radio kits, mostly based around a core Atmel processor control unit. Having previously built a six-band version of his Ultimate 3S QRP WSPR beacon transmitter (which can do so many more modes than that!) and the QLG1 GPS receiver to govern it, and having a need for an effective way to replace crystals in vintage military radios in order to test conversion to amateur bands, I decided to buy the QRPlabs Si5351A based VFO/Sig Gen kit.

The kit comprises a 2x16 backlit LCD module, the Si5351A DDS module, and the QCV control module. This is exactly the same as the U3S, the only difference being the firmware version of the processor chip, and some of the discrete components. The DDS module is capable of working up to at least 150MHz, and in this kit, produces two independent  outputs. One output is fixed frequency, set up in presets in the firmware, the other the VFO output. The above photo shows the PCBs after populating.

This kit comes with a rotary encoder with a push button switch action. This was the only place I went astray during the build, as I managed to mix up one of the encoder lines with one of the switch contacts, resulting in some rather odd things happening when trying to test!

After a good rummage around, I came up with a metal enclosure for it. I had hoped to use a die-cast box, but all my stock were either too small or way too big! But I had this sheet aluminium box kicking about Its a slightly odd enclosure as it has the seams of one half on the outside of the other, rather than hidden inside.

What took most of the build time was the marking out, drilling and cutting of the box.

 Somehow I managed to drill the encoder hole far too wide, but the washer supplied with it covers this error up,

Unfortunately I havent been able to complete the build today, as I discovered that I dont have any panel mount push buttons in stock, nor any suitable stand offs to mount the unit into the case with! Ive now ordered these, but they may take several weeks to arrive!

A quick test showed all to be well. One thing I discovered though which I hadnt expected, was that the firmware includes backlight brightness control. I had already installed the link for full brightness as I didnt expect this feature, which is used in the U3S, to be in this firmware. So, that link was removed and the correct link for processor control of the brightness added.

Im so pleased with how well this has gone together that I decided I just had to build another of Hans' kits! So, since I needed to measure the QLG1 GPS units dimensions (as ive asked a work mate to 3D print me a box for it!) and this meant pulling out the U3S, I decided that what I really need is a touch more power for WSPR ... so now the QRPlabs 5W HF PA kit is on order, along with an upgraded firmware chip for my U3S, which will control the PA and also give me the ability to control an antenna changeover relay. The HF PA kit has the same PCB form factor as the rest of the kits PCBs, so effectively just 'bolts on'.

Tuesday, 22 January 2019

HT Chain issues

I have a requirement to replace the mechanical switch shown here, that selects one of 18 different HT levels 0-80v from a potential divider chain, with electronic switching controlled by a 3v logic port expander.

Im currently thinking p-channel MOSFETs for the HT switch, but these need biasing from the HT line they are on, which I think would kill the logic gates! So, I also need a driver device that will isolate the 3v logic control (MCP23017) from the HT and the switch.

Sunday, 20 January 2019

PRC-344 Alignment - Working Fully

The Clansman PRC344 is now probably as good, electrically, as it will get.

Having removed the Direct Current Amplifier module3, in order to disable the radios ability to transmit (since there is absolutely no legal way of using the transmitter, yet lots of ways to inadvertently send a signal that at best could get you prosecuted and at worse endanger an aircraft!), I could happily ignore all the alignment process relating to the transmitter.

So this evening I took the time to run through the alignment processes for the power supplies and the receiver. The only part of this that gave any problem was the AGC alignment. This required setting one control for 350mV peak waveform on the oscilloscope, and 0v on the voltmeter. However, although I could hear the receiver being affected by the adjustments, I couldnt get anything like the specified waveform levels!

*I have now realised that I was monitoring for the 0v on the wrong test point! It may be that I have to do this section again sometime!

But, the radio does now seem to be performing as specified. One downside to this being that with the mute setting as per the manual of -108dBm, the background level of QRM within my QTH is enough on many channels to open the mute!

The final test required, ive just completed. This is rather a frivolous aspect, but I wanted it to work, so ive tested the REMOTE and I/C settings using a Remote Handset and a few meters of DON10 telephone wire. But, with this working, it means that I could stick the radio up on the workshop roof and run telephone wire into my warm comfortable work position to monitor while I get on with something else.

Pye PF8 - Stumped by the filters

With the band specific capacitors replaced, Ive managed to peak the transmit output of the PF8 up to about 170mW. This is still a far cry from the specified 500mW. Likewise, the receiver barely peaks higher than -85dBm.

In order to make certain this was a filter issue, I have bypassed the 3-pole input helical filter - and the receive sensitivity shot up!

But now I have a big problem. Not only are the correct filters simply not available, but the ones fitted cannot be modified to work better either. In fact, its near impossible to remove them anyway without destroying them (as I found when attempting to take one off my 'donor' unit!)

So, what to do? Well, pretty much all I can do is either find some 2- and 3-pole helical filters for 430MHz that will fit, probably impossible, or at least prohibitively expensive, or think up some way to replace the helical filters with small home brew band pass filters.

All this just so I can own a working, genuine Bodie and Doyle CI5 walkie-talkie!

Pye PF8 Conversion Progress

It has taken me a little longer than anticipated to get to this stage, as I replaced the crystals and did the initial tune up for 70cm way back in 2016! But today I finally embarked on the task of changing the band specific capacitors,

In order to replace these caps, the side of the radio needed to be removed, this involved also desoldering the 'call' switch, as well as removing the bolts through the PTT buttons. Whenever I work on something as delicate as this, I take copious photos - so I can see where everything goes back to!

The first two capacitors are along the very edge of the PCB, and up against one of the filters.

C36, 2nd from left (brown) and C35, 4th from left (orange) fitted
If fitting these was fiddly, then the next three were a nightmare! These three were in-board, and surrounded by other much taller, and very delicate parts. Replacing these took all the skill I could muster, and a lot of will power to ignore the pain of burning fingers!

C45 black left of power transistor; C47 above transistor; C11 left of green cap
 A minor disaster awaited me - in desoldering C11, I inadvertently also desoldered C13 and R16! Both of which fell out. It took a lot of desoldering to clean up and open the PCB holes in order to be able to get them back in!

PCB holes cleared to replace C13 and R16, and fit new C11
 All this took over two hours! I then had to reposition and secure the wiring loom on the track side, refit the screening plate, and ensure that the tiny coax that goes to one leg of C47 was correctly resoldered.

The wiring loom and shielding
 So now im in a position to test again. All being well, the set will power up! If I get past that milestone, im hoping that at the very least I will now be able to bring the Tx power up closer to the designed 500mW. Whether I get any improvement on receive we'll have to wait and see.